Analysis & Opinion

AMMAN (Reuters) - Syrian forces pressed on with their offensive against rebels in the largest city Aleppo after the prime minister fled the country, denouncing the "terrorist regime" of Bashar al-Assad.

The defection of Riyad Hijab - who like most of the opposition hails from the Sunni Muslim majority - was a further sign of the isolation of Assad's government around an inner core of powerful members of his minority Alawite sect.

Opposition figures, buoyant despite setbacks in recent weeks of fighting around Damascus and Aleppo, spoke of an extensive and long-planned operation to spirit Hijab and his large extended family across the border to Jordan.

"I announce today my defection from the killing and terrorist regime and I announce that I have joined the ranks of the freedom and dignity revolution," Hijab said in a statement read by a spokesman on Al Jazeera television on Monday. He declared himself "a soldier in this blessed revolution".

A spokesman for U.S. President Barack Obama hailed Hijab's defection as a sign that the 40-year rule of Assad's family was "crumbling from within" and said he should step down.

The security forces have overwhelming superiority in firepower, which they have wielded against lightly armed rebels who poured into the two main cities, Damascus and Aleppo, in the past month.

The rebels gathered momentum last month, attempting to seize the two cities after an audacious bomb attack killed four members of Assad's inner circle. But a government counteroffensive has been devastating, with troops largely recapturing Damascus and using helicopters and tanks to hammer rebels who retain control of parts of Aleppo.

The war increasingly has divided the region along its sectarian faultline, pitting the mainly-Sunni rebels, who are backed by regional Sunni-led powers Turkey and the Gulf Arab states, against a government that is backed by Shi'ite Iran.

Iran has expressed worry over the fate of more than 40 Iranians Tehran says are religious pilgrims kidnapped by rebels off a bus in Damascus while visiting Shi'ite shrines.

The rebels say they suspect the captives were troops sent to aid Assad. A rebel spokesman in the Damascus area said on Monday three of the Iranians had been killed by government shelling, and the rest would be executed if the shelling did not stop.

Hijab's defection was the latest sign of Sunnis abandoning Assad, but there has been no sign yet that members of his mainly Alawite ruling inner circle are losing their will to fight on.

"Defections are occurring in all components of the regime save its hard inner core, which for now has given no signs of fracturing," said Peter Harling at the International Crisis Group think-tank.

"For months the regime has been eroding and shedding its outer layers, while rebuilding itself around a large, diehard fighting force," he said. "The regime as we knew it is certainly much weakened, but the question remains of how to deal with what it has become."

ALEPPO DISTRICT WHITE WITH DUST

Rebels in districts of Aleppo visited by Reuters journalists in recent days seemed battered, overwhelmed and running low on ammunition after days of intense shelling of their positions by tanks and heavy machinegun fire from helicopter gunships.

Syrian army tanks shelled alleyways where rebels sought cover as a helicopter gunship fired heavy machineguns. Snipers ran on rooftops targeting rebels. Women and children fled the city, some crammed in the back of pickup trucks, while others trekked on foot, heading to relatively safer rural areas.

The main focus of fighting in Aleppo has been the Salaheddine district. Once a busy shopping and restaurant district, it is now white with dust, broken concrete and rubble.

Tank shell holes gape wide on the top of buildings near the front line, and homes of families have been turned into look-outs and sniper locations for rebel fighters. Large mounds of concrete are used as barriers to close off streets. Lampposts lie horizontally across the road after being downed by shelling.

A bomb hit the Damascus headquarters of Syria's state broadcaster on Monday but injuries were minor and transmission continued.